In the production of automotive vehicles it is a known practice to assemble tires and wheels at high rates of assembly and then to balance the tires and wheels on an automated machine which also operates at a high rate, say 9 or 10 seconds per tire. Such machines have a station to center a tire/wheel assembly, a balance measuring station, and a marking station where the position and amount of unbalance is marked on the tire, and a transfer mechanism for sequentially moving the wheel assembly (hereinafter called a "wheel") from one station to another.
The balance measuring station typically includes a rotatable spindle having an air pressure operated collet which holds the wheel hub attachment plate or spider at its central opening, a motor driven drum which is pressed against the outer periphery of the tire to spin the wheel, and a vibration sensor for detecting the effect of unbalance. In addition, a balance measuring circuit determines from the detected vibration the amount and position of the unbalance.
It is known, as shown in FIG. 1, to impart rotation to the wheel 10 on the spindle 24 by engaging the tire periphery by a motor driven wheel or drum 12. Typically the drum 12 is mounted on one end of an arm which pivots about the other end for movement of the drum from a home position to an operating position against the tire 14 where it presses radially against the tire with sufficient force F to sustain a transfer of torque from the rotating drum to the tire. That radial force F is transferred by the tire to the wheel rims 18 as forces F' and thence through the hub attachment plate 20 to the spindle 24. Since the rims 18 are spaced from the hub attachment plate 20 which engages the collet 22 on the spindle 24, the force F' on each rim creates a moment about the spindle. Because the wheel usually has its hub attachment plate 20 offset from the mid-plane of the wheel, the forces F' are unequally spaced from the plate 20 and hence the moments are not equal. The differential of the moments creates a net moment M on the wheel which tends to tilt the wheel from its desired attitude which is normal to the axis of the spindle. The air operated collet 22 which frictionally engages the center hole of the attachment plate must overcome the net moment M if the wheel is to be secure on the spindle. Experience with such machines proves that where the net moment is significant, the wheel can overcome the collet force and walk up the collet during tire rotation.
It is also known, as shown in the U.S. Pat. No. 4,191,055 to Orem et al entitled "Dynamic Balance Determining System", to rotate the wheel by two powered rotating drums pressed against the outer tire periphery. There, the drums are located on opposite side of the conveyor that carries the wheels and are supported by arms pivoted at positions downstream of the drums; that is the pivot points of the arms are both at one side of the wheel in the direction of conveyor movement. As a result, the drums move in arcuate paths between home positions and tire driving positions. For one wheel size the drums will contact the wheel at diametrically opposite points. In that particular case the radial force of each drum against the wheel will offset the force of the other drum and there will be no net moment tending to lift the wheel from the collet. However, for larger or smaller wheels, the contact points of the drums with the wheel will not be at diametrically opposite locations and the radial forces will not offset one another; rather, both drums will contribute to a moment tending to lift the wheel from the collet.